Field of the Invention
The present invention relates to a lithium ion capacitor.
Description of the Related Art
Electrical double-layer capacitors using non-aqueous electrolytic solution offer high electrolytic decomposition voltage of solvent and therefore achieve high voltage resistance, and are able to store a large amount of energy as a result. In particular, lithium ion capacitors have greater capacitance per unit volume than electrical double-layer capacitors, and they also last longer than lithium ion secondary batteries when the discharge depth of their negative electrode is reduced or a polarizing positive electrode material is used. In recent years, there are calls for lithium ion capacitors that achieve lower internal resistance at low temperatures, while ensuring reliability in a high-temperature environment. In terms of characteristics, lithium ion capacitors are subject to rise in internal resistance at low temperatures due to reduced electrolyte dissociation in the electrolytic solution and rise in the viscosity of the electrolytic solution; at high temperatures, their reliability is affected probably due to deterioration in cell characteristics as a result of decomposition of PF6− and other anions constituting the electrolyte and consequent generation of hydrogen fluoride and other decomposition products.
To solve the aforementioned problems, Patent Literature 1, for example, proposes a lithium ion battery that uses an electrolytic solution constituted by a mixture of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), being used as an electrolyte, and a mixed solvent of cyclic carbonate and chained carbonate, being used as a solvent for non-aqueous electrolytic solution. Patent Literature 2 proposes a lithium ion battery that uses an electrolytic solution whose electrolyte is LiFSI, and whose solvent for non-aqueous electrolytic solution is a mixed solvent of chained carbonic acid ester, cyclic carbonic acid ester, chained ester, and cyclic ether. Patent Literature 3 proposes a lithium ion battery that uses an electrolytic solution whose electrolyte is LiFSI, and whose solvent for non-aqueous electrolytic solution is a mixed solvent of cyclic carbonate and chained carbonate.